This application is based upon Japanese Patent Application No. 2000-173796 filed on Jun. 9, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device having a trench, and more particularly of a semiconductor device having a buried gate.
2. Related Arts
Recently, a semiconductor device with an electrical isolation (trench isolation) structure or a semiconductor device with an embedded gate (trench type gate) has been proposed in accordance with improved integration degrees of various semiconductor devices.
A semiconductor device with a trench is formed by a method shown in FIGS. 13A to 13E as one method of prior art.
First, an opening portion 2a is formed in a mask member 2 for forming a trench by patterning the mask member 2 formed on a silicon substrate 1 as shown in FIG. 13A. Then, a concavity portion 3 having a bowl shape is formed at a portion where a trench opening portion is to be formed by isotropic etching as shown in FIG. 13B. After that, a trench 4 is formed in the silicon substrate 1 by anisotropic etching as shown in FIG. 13C.
Subsequently, formation of a sacrificial oxide film, removal of the sacrificial oxide film, and removal of the mask member 2 (see FIG. 13D) are performed successively. After that, an insulation film 5 is formed in the trench 6 as shown in FIG. 13C. Then, the trench 4 is filled with polycrystalline silicon 6, and a part of the polycrystalline silicon 4 disposed outside of the trench 4 is removed as shown in FIG. 13E.
According to this method, an angle at an opening portion of the trench 4 is prevented from becoming about 90xc2x0 because a shape of the opening portion of the trench 4 is improved by the isotropic etching. However, as shown in FIG. 13C, a portion A contacting a side face of the trench 4 becomes a sharp edge because of the shape of the concavity portion 3, or a portion B disposed at a corner of the trench 4 becomes a sharp edge.
Shapes of these portions A and B are not easily improved after a thermal oxidation step is conducted. When the sharp edge of the portion A or B remains, insulation characteristics of the insulation film 5 formed in the trench 4 is deteriorated since the portion A or B becomes a concentrating point of electric field.
Moreover, thermal treatment is conducted while an etching damage layer 7 remains (see FIG. 13C), whereby a defect layer is easily formed on an inner surface of the trench 4. Therefore, the insulation characteristics of the insulation film 5 formed in the trench 4 may be deteriorated in a case that the insulation film 5 is formed by oxidizing the remaining damage layer 7 or the defect layer.
Another method is shown in FIGS. 14A to 14E. An opening portion 2a is formed in a mask member 2 as shown in FIG. 14A. Then, a trench 4 is formed in a silicon substrate 1 by anisotropic etching as shown in FIG. 14B. Subsequently, a shape at an opening portion of the trench 4 and a shape at a bottom portion of the trench 4 are improved by CDE (Chemical Dry Etching) treatment simultaneously as shown in FIG. 14C. After that, formation of a sacrificial oxide film, removal of the sacrificial oxide film, and removal of the mask member 2 (see FIG. 14 D) are performed successively. Then, after an insulation film 5 is formed in the trench 4, the trench 4 is filled with a polycrystalline silicon 6 as shown in FIG. 14E.
According to this method, formation of the portions A and B each having a sharp edge due to the shape of the concavity portion 3 shown in FIG. 13C and the concentration of electric field caused by the portions A and B are restrained.
However, since etching by the CDE treatment is isotropic dry etching, etching amount of the silicon in the trench 4 becomes large to round the opening portion and the bottom portion of the trench 4 sufficiently. As a result, a shape of the trench 4 fluctuates greatly. In addition, a polymer 8 composed of Teflon is formed and remains in the trench by the CDE treatment. The polymer 8 also deteriorates the insulation characteristics of the insulation film 5.
On the other hand, when the etching amount of the silicon is small, the shape of the trench 4 is not improved sufficiently, and the damage layer 7 remains in the trench 4, which deteriorates the insulation characteristics of the insulation film 5 as described above.
The present invention is devised to solve the above problems, and has an object to provide a manufacturing method of a semiconductor device having a trench in which an insulation layer with a high withstanding voltage characteristics and high quality can be formed.
According to a first aspect of the invention, after a trench is formed in a semiconductor substrate by anisotropic etching, a wet process is performed so that the semiconductor substrate is etched by a mixed solution containing hydrofluoric acid and nitric acid, whereby a shape of the trench is improved and a damage layer remaining in the trench is removed. Thus, an insulation layer, a semiconductor layer or the like is formed on a surface of the trench, which is suitable for forming the layer of the insulation film, the semiconductor film or the like. Therefore, quality of the layer of the insulation film, the semiconductor film or the like becomes fine, whereby a semiconductor device can have good electrical characteristics.
According to a second aspect of the invention, a high impurity concentration region is formed in a surface region of the semiconductor substrate. The wet process depends on a concentration of impurities in the semiconductor substrate.
Therefore, an etching rate at the surface region of the semiconductor substrate is higher than that at a deep portion of the semiconductor substrate in the trench deeper than the surface region. Thus, a shape at an opening portion of the trench is modified from a right-angled shape after the anisotropic etching is performed into a rounded shape.
According to a third aspect of the invention, after the trench is formed in the substrate by the anisotropic etching, short-time heat treatment is performed, whereby crystallinity of the trench is improved. That is, the crystallinnity of the trench is restored.
According to a fourth aspect of the invention, after the wet process is performed, short-time heat treatment is conducted successively, whereby crystallinity of an inner surface of the trench is improved.
According to a fifth aspect of the invention, a semiconductor device having good electrical characteristics is achieved by a manufacturing method described below.
Namely, a trench is formed in a semiconductor substrate in a depth direction thereof through a mask member, which is formed on the semiconductor substrate, having an opening portion. Then, a polymer, generated in forming the trench, remaining in the trench is removed. Next, a wet process is performed so that the semiconductor substrate is etched by a mixed solution containing hydrofluoric acid and nitric acid. Thus, taper portions are formed on an opening portion and a bottom portion of the trench simultaneously by one feature inhering in the mixed solution that an etching rate depends on a face orientation of the semiconductor substrate, i.e., by a face orientation dependency of the etching rate.
Moreover, a damaged layer formed in the trench by the anisotropic etching for forming the trench, which may deteriorate quality of an insulation layer to be formed in the trench, can be removed by the other feature inhering in the mixed solution that an etching rate of the damaged layer in a semiconductor is different from that of a non-damaged layer in the semiconductor, i.e., by etching selectivity in the semiconductor.
Then, after a thermal oxidation film is formed in the trench as a sacrificial oxide film by thermal oxidizing treatment, the sacrificial oxide film is removed, whereby shapes at the opening portion and the bottom portion of the trench are rounded. Subsequently, the mask member is removed, an insulation film is formed in the trench, and then, the trench is filled with a polycrystalline silicon.
The trench formed through the steps described above can be applied to a trench for insulated isolating member that is formed around an element forming region to isolate the element from other elements, or applied to a trench for a trench gate type MOSFET or IGBT in which the insulation film is used as a gate insulation film, and the polycrystalline silicon is used as a gate electrode.
According to a sixth aspect of the invention, a semiconductor device having good electrical characteristics is achieved by a manufacturing method described below.
A trench is formed in a semiconductor substrate in a depth direction thereof through a mask member, formed on the semiconductor substrate, having an opening portion. Then, a polymer, generated in forming the trench, remaining in the trench is removed.
Then, short-time heat treatment is conducted successively, whereby crystallinity of an inner surface of the trench is improved. Therefore, it is preferable to apply the trench formed by the method described above to a trench for insulated isolating member that is formed around an element forming region to isolate the element from other elements, or to a trench gate type MOSFET or IGBT in which an insulation film formed on an inner surface of the trench is used as a gate insulation film, and a polycrystalline silicon filling the trench is used as a gate electrode.
According to a seventh aspect of the invention, a first conductive type layer is formed on one surface of a semiconductor substrate. A second conductive type region is formed in the first conductive type layer, and a first conductive type region is formed in the second conductive type region.
Therefore, when the method as described in the fifth aspect of the present invention is applied to this substrate, in addition to the effects by the two features of the mixed solution described above, an etching rate is varied in accordance with an impurity concentration profile of the first conductive type layer exposed in the trench by another feature inhering in the mixed solution that an etching ratio depends on an impurity concentration of the semiconductor substrate, i.e., by impurity concentration dependency of the etching rate, so that the shape of the opening portion of the trench is tapered so as to form a convexity. Moreover, the damaged layer in the trench is removed, and the inner surface of the trench is smoothed.
According to an eighth aspect of the invention, a first conductive type layer is formed on one surface of a semiconductor substrate. A second conductive type region is formed in the first conductive type layer, and a first conductive type region is formed in the second conductive type region.
Then, a trench is formed in a semiconductor substrate in a depth direction thereof through a mask member, formed on the semiconductor substrate, having an opening portion. Then, a polymer, generated in forming the trench, remaining in the trench is removed.
Next, short-time heat treatment is conducted, whereby crystallinity of an inner surface of the trench is improved. Therefore, it is preferable to apply the trench formed by the method described above to a trench for insulated isolating member that is formed around an element forming region to isolate the element from other elements, or to a trench gate type MOSFET or IGBT in which an insulation film formed on an inner surface of the trench is used as a gate insulation film, and a polycrystalline silicon filling the trench is used as a gate electrode.
According to a ninth aspect of the present invention, in the method described in the fifth aspect of the present invention, it is preferable that short-time heat treatment is performed before the thermal oxidizing treatment for forming the thermal oxidation film since crystallinity of an inner surface of the trench is improved (restored).
It is preferable in the method described in the first aspect of the present invention that the opening portion of the trench has a long shape (a rectangular shape) since the face orientation dependency of the etching ratio as the one of features inhering in the mixed solution described above works sufficiently, so that the taper portions are formed in the opening portion and the bottom portion of the trench preferably.
It is preferable that a volume ratio of the hydrofluoric acid in the mixed solution with respect to the nitric acid is set to a range 1:50 to 1:300 to effectively obtain the effects caused by the features of the mixed solution described above.
It is preferable that the mixed solution is diluted with pure water to obtain the effects obtained by the mixed solution described above while reducing an amount of the nitric acid.
It is preferable that the mask member and the thermal oxidation film are removed simultaneously.
It is preferable that the thermal oxidizing treatment for forming the thermal oxidation film is performed at 1100xc2x0 C. Thus, the thermal oxidation film is formed so that the shape of the trench is modified to have a rounded shape effectively.
It is preferable that the thermal oxidizing treatment for forming the thermal oxidation film is performed after the mask member is removed.
It is preferable that the thermal oxidizing treatment for forming the thermal oxidation film is performed at 900xc2x0 C. in atmosphere containing water vapor.
It is preferable that the insulation film formed on the inner surface of the trench is composed of a laminated layer containing a silicon oxide film and a silicon nitride film to improve electric characteristics thereof.
It is preferable that the silicon oxide film composing the insulation film includes a laminated silicon oxide film composed of a silicon oxide film formed by low-pressure CVD using TEOS (Tetra Ethyl Ortho-Silicate) as a raw material and a thermal oxidation film formed by thermal oxidation, so that a thickness of the oxide film is uniformed throughout by the CVD silicon oxide film, and an interface of the oxide film is improved by the thermal oxidation film.
It is preferable that the silicon oxide film composing the insulation film is formed by a step in which a silicon oxide film is formed by CVD method, and then, this CVD oxide film is thermally oxidized to densify and to make an interface between the silicon oxide film and a semiconductor fine.
It is preferable that the silicon nitride film composing the insulation film is formed by low-pressure CVD to densify, so that insulating capacity of the insulation layer is improved.
It is preferable that the first conductive type region is formed by implanting arsenic ions into the second conductive type region at 8xc3x971014/cm2 or less, so that quality in a insulating layer formed on the first conductive type region is preventing from deteriorating.
It is preferable that the short-time heat treatment is performed at 800xc2x0 C. or more in an atmosphere of hydrogen, so that the crystallinity of a semiconductor exposing on the trench is improved.
It is preferable that the short-time heat treatment is performed under reduced pressure, so that the crystallinity of a semiconductor exposing on the trench is improved, whereby quality of the insulation layer is improved.
It is preferable that the semiconductor substrate has a (110) face orientation, a variation of which is in a range of xc2x13xc2x0, and the trench is formed so that an extending direction of the trench is in a range xc2x13xc2x0 with respect to a (100) face.
It is preferable that when the surface of the semiconductor substrate is inclined with respect to the (110) face, a long side of the trench formed by anisotropic etching is in parallel with an inclined direction being in parallel with the surface and being perpendicular to a non-inclined direction parallel with respect to the surface of the semiconductor substrate and the (110) face. Accordingly, in the trench, oxide films formed on a bottom, a corner in the vicinity of the bottom, and the opening portion are formed thicker than that formed on a sidewall of the trench. Namely, thickness of the oxide films formed on a corner at the opening portion and the corner peripheral of the bottom of the trench is prevented from being formed to be thin. In addition, dry oxidizing treatment is better to form thick oxide films on the corner at the opening portion and the corner peripheral of the bottom of the trench.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.